It is not very clear what position the sharks sleep in. Some species lie on the seafloor continuing to actively pump water through their gills and checking the surrounding situation with their eyes open. While resting, these sharks suck in water not through the nostrils, but through openings near the eye, called spiracles.
If this were not the case, leaning against the ocean floor, they would suck up more sand than water. Many scientists believe that the presence of blowholes in sharks is mainly due to this reason. In the dogfish, on the other hand, the spinal cord, not the brain, coordinates the swim.
This peculiarity allows this species to continue to move even during sleep. Shark sleep also occurs to resemble that of dolphins. In this case only one part of the brain rests, while the second part partially guarantees a state of consciousness.
Energy conservation characterizes sleep in sharks, study published on the Biology letters, explained: "Sharks represent the earliest group of jawed vertebrates and as such, they may provide original insight for understanding the evolution of sleep in more derived animals.
Unfortunately, beyond a single behavioral investigation, very little is known about sleep in these ancient predators. As such, recordings of physiological indicators of sleep in sharks have never been reported. Reduced energy expenditure arising from sustained restfulness and lowered metabolic rate during sleep have given rise to the hypothesis that sleep plays an important role for energy conservation.
To determine whether this idea applies also to sharks, we compared metabolic rates of draughtsboard sharks (Cephaloscyllium isabellum) during periods ostensibly thought to be sleep, along with restful and actively swimming sharks across a 24 h period We also investigated behaviors that often characterize sleep in other animals, including eye closure and postural recumbency, to establish relationships between physiology and behavior.
Overall, lower metabolic rate and a flat body posture reflect sleep in draughtsboard sharks, whereas eye closure is a poorer indication of sleep. Our results support the idea for the conservation of energy as a function of sleep in these basal vertebrates."